Development of multimode optical imaging technology has centered on coherent heterodyne or homodyne detection processes. Heterodyne (or homodyne) detection of electromagnetic fields is based on the interferometric mixing of a coherent reference field or “local oscillator” with a coherent object field in a manner that captures and preserves amplitude and phase information. The fundamental advantages of coherent over non-coherent detection processes include the filterless separation of image fields from clutter and background, controlled noise bandwidths, and coherent amplification. In practice, coherent processes enable system designers to exploit a broad range of multimode detection building blocks, including multi-channel access, amplitude/phase detection, and range/Doppler discrimination.
Practical implementation of coherent detection technology at short wavelengths is hindered by the intrinsic difficulty in matching the spatio-temporal states of object and reference fields. Spatio-temporal states characterize the time-varying spatial amplitude, phase, and polarization distribution of a propagating field where spatial states become critical when the transverse dimensions of the receiver aperture are large compared with the field's wavelength. A stable coherent detection process requires precise temporal and spatial matching of the object and reference field states. The progressive loss in spatio-temporal coherence between object and reference fields eventually leads to systematic phase and amplitude degradations in the detection process resulting in loss of conversion efficiency, corrupted image retrieval, and increased noise bandwidth. Causes of spatio-temporal mismatches include transceiver source/reference drift and fluctuations, atmospheric turbulence, scattering from rough surfaces, relative motion. A description of scalar projection process and relevant experimental apparatus may be found in U.S. Pat. No. 5,875,030, B. J. Cooke and A. Galbraith, Method and Apparatus for Coherent Electromagnetic Field Imaging through Fourier Transform Heterodyne. 
Scalar projection concepts led to hybrid fields that provide a mechanism through which spatial and temporal matched filter conditions required for short wavelength coherent detection are realized. In essence, hybrid fields superpositions of two or more matched fields with coherent detection uniqueness introduced through the selective modulation of one set of field states relative to a matching set of field states. Modulation is necessary because the modulation creates a distinctly detectable interferometric signature under square-law detection. Applicable hybrid field modulation formats include: amplitude, phase, frequency, and polarization. Successive experiments demonstrated the feasibility of generating and manipulating hybrid fields for efficient coherent detection or range, Doppler, and multi-channel imaging. Further, the experiments established the concept of synthetic wavelengths and stationary phase processes, which are vital to understanding synthetic aperture image degradation at short wavelengths. Hybrid fields and the associated experimental apparatus are described in more detail in U.S. Pat. No. 7,417,744, B. J. Cooke and D. C. Gunether, Coherent Hybrid Electromagnetic Field Imaging. 